Synchronous Optical NETwork (SONET) and Synchronous Digital Hierarchy (SDH) describe two families of closely related and compatible standards that govern interface parameters; rates, formats and multiplexing methods; operations, administration, maintenance and provisioning for high-speed signal transmission. SONET is primarily a set of North American standards with a fundamental transport rate beginning at approximately 52 Mb/s (i.e., 51.84 Mb/s), while SDH, principally used in Europe and Asia, defines a basic rate near 155 Mb/s (to be precise, 51.84×3=155.52 Mb/s). From a transmission perspective, together they provide an international basis for supporting both existing and new services in the developed and developing countries.
For transmitting data, SDH and SONET use frame formats transmitted every 125 μs (8000 frames/sec). Because of compatibility between SDH and SONET, their basic frames are similarly structured, but differ in dimension which fact reflects the basic transmission rates of 155.52 and 51.84 Mb/s, respectively. To be more specific, a basic frame format of SDH is 9 rows of 270 bytes, or 2430 bits/frame, corresponding to an aggregate frame rate of 155.52 Mb/s. For SDH systems, the mentioned basic frame transmitted at the rate 155.52 Mb/s forms the fundamental building block called Synchronous Transport Module Level-1 (STM-1). For SONET systems, the basic frame has dimensions of 9 rows by 90 byte columns and, being transmitted at the rate 51.84 Mb/s, forms the appropriate fundamental building block called Synchronous Transport Signal Level-1 (STS-1).
Lower rate payloads (data portions transmitted at rates smaller than the basic ones) are mapped into the fundamental building blocks, while higher rate signals are generated by synchronously multiplexing N fundamental building blocks to form STM-N signals (in SDH) or STS-N signals (in SONET).
Each basic frame in SONET or SDH comprises an information portion called Information Payload and a service portion called Overhead (OH), the latter being subdivided into a number of areas of overhead bytes (for example, Path Overhead—POH, Transport Overhead—TOH) predestined for various service and control functions. One of such areas is a column of Path Overhead (POH) usually residing within the Information Payload area. POH supports performance monitoring, status feedback, signal labeling, user channel and a tracing function in a path. This overhead is added and dismantled at or near the service origination/termination points defining the path, and is not processed at intermediary nodes.
The SDH multiplexing structure, defined in ITU-T Recommendation G.707(03/96), comprises so-called virtual containers serving to combine lower rate payloads such as by mapping into these containers and adding POH. The combined payloads fitted with POH are further aligned and multiplexed in order to form an STM-N signal. The STM-N signal can be obtained either by multiplexing AU-3 signal (accepted also in SONET) by 3N, or by multiplexing N signals AU-4. AU-4 is formed by adding pointers to a VC-4 signal (virtual container level-4). Similarly, the AU-3 signal is formed from VC-3 by adding AU-3 pointers. Lower level signals TU-11, TU-12, TU-2 and TU-3, which are formed by adding respective pointers to lower level virtual containers VC-11, VC-12, VC-2 and VC-3, in their turn serve as components for composing the higher level virtual containers VC-3 or VC-4.
According to ITU-T Recommendation G.707(03/96), the POH of virtual containers consists of a plurality of bytes, most of which are used for end-to-end communication. For example, when an error is detected at a path terminating equipment (PTE), an Alarm Indication Signal (AIS) is to be transmitted to the downstream network elements, and a signal of Remote Defect Indication (RDI)—towards the upstream network elements. The AIS is stated to be in the form of all “1”-s sent in the payload associated with this path, and the RDI is a code sent in the path status byte G1 assigned in the POH for this purpose. In SONET, the Transport overhead layer which is responsible for transport through the network, is broken into two parts—Line Overhead (LOH) and Section Overhead (SOH). Section overhead (SOH) is that overhead necessary for reliable communication between network elements such as terminals and regenerators. Line overhead (LOH) was established to allow reliable communication of necessary information between more complicated network elements such as terminals, digital cross-connects, multiplexers and switches. In SDH, no Transport overhead is directly defined. However, any SDH basic frame comprises a Multiplex Section Overhead (MSOH) being analogous to LOH in SONET, and a Regenerator Section Overhead (RSOH) which play the part of SOH in SONET. FIG. 4 schematically illustrates an STM-1 frame with its overhead sections. It is therefore considered, that the layer of Transport overhead in SDH is actually formed by the mentioned MSOH and RSOH. POH column is considered a part of STM-1 payload. There is also a row of overhead bytes bearing information on so-called AU-pointers (administrative unit pointers) which are considered to belong neither to TOH nor to POH, and are analyzed separately. Some of overhead bytes (for example, a number of SOH bytes) are reserved for future use.
During decoding and checking of a SONET or SDH signal transmitted in a path, it is generally “stripped down” in layers by a control system, first decoding and checking the section information, then the line information and thereafter the path information. At each step, error checking is provided and errors (if detected) must be indicated either to the local control system, or to the originating path terminal element to inform about troubles in the path.
Such a concept has a problem that many network elements positioned upstream and downstream of a particular network element, where a trouble has been first revealed, receive information on this trouble (as well as other troubles found in the path) without knowing its source. As a result, the network manager system (NMS) should establish additional communication to the network elements for exactly determining which of them reported each of the particular faults. These extra interactions require extra time and complicate the network operation. It goes without saying that exact information on troublesome sections of the network is extremely important for its management.
Standards and Recommendations dealing with SDH and SONET transmission systems do not describe or suggest particular solutions to the problem outlined above.
Another problem, which is known to those skilled in the art, stems from the fact that important information to be passed via a network is quite often transmitted via at least two alternate routes. One of the alternate data streams is usually to be selected at points of cross-connect and destination. In such cases, where a data stream (trail) is transmitted via two or more alternative paths, information on the respective quality of transmission would be of great importance: whenever one stream is to be selected out of those transmitted via the alternative paths, the information characterizing a so-called “trouble rank” or “data quality” of these alternative paths could be used.
U.S. Pat. No. 5,710,759 describes a switch protection arrangement. Digital information transmitted via an SDH network arrives to the input of the switch in the form of a data frame. This input information in the frame is reorganized within the switch and checked for parity by using a V4 overhead byte of the frame before being issued from the switch. The procedure is proposed for selecting one of receive signals formed at the switch via two alternative channels by checking the presence or absence of a code error in each of the signals. The arrangement described in the US patent is intended for checking the switch equipment, and not the transmitted data. Namely, the test data byte is used to assist in determination of whether the switch paths are routed and operating correctly.
Recommendations Nos. ITU-T G.707 and G.783 of Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) do not provide technological solutions as of how data streams transmitted via alternative paths could be sorted.